1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a memory medium therefor.
2. Related Background Art
Recently there is being standardized the so-called digital watermarking technology for embedding, in image information such as a photograph or a picture, additional information consisting of so-called copyright information such as the name of author or the presence/absence of permission for use in visually easily unrecognizable manner for dissemination through networks such as internet.
Also with the progress in image quality of the copying apparatus and the printer, in order to avoid illegal forging of banknotes, stamps, valuable documents etc., there is known a technology of embedding additional information such as the information indicating the number of the output device in the image, in order to specify the output device and the number thereof from the image printed on paper.
For example, the Japanese Patent Application Laid-Open No. 7-123244 proposes a technology of embedding additional information in the high frequency range of the color difference component and saturation component which is of visually low sensitivity.
However, the above-described conventional technology has been associated with the following drawbacks. FIG. 13 illustrates embedding of general additional information in the digital watermarking technology. In the image information A, the additional information B is embedded by an adder 1301 to obtain multiplexed information C. FIG. 13 shows an example of embedding the additional information in the real space area of the image information. If such multiplexed information can be disseminated without image processing such as various filterings or encoding such as non-reversible compression, the additional information B can be easily extracted from the multiplexed information C even in the conventional technology. In the image information distributed on the internet, such extraction is possible through a digital filter for improving the image quality such as edge enhancement or smoothing, as long as there is certain noise resistance.
However, let us assume a case of printing the multiplexed image by an output device such as a printer and extracting the additional information from the print, employing a printer of representing ability of two to several gradation levels per color. Among the recent ink jet printers, there is known a device capable of representing several gradation levels by employing inks of lower dye concentration or variably controlling the output dot diameter, but the gradation of the photographic image cannot be reproduced unless pseudo gradation process is adopted.
Thus, in the aforementioned case of printer output of the multiplexed information employing the digital watermarking technology shown in FIG. 13, the multiplexed information C at first changes to quantized information D by a pseudo gradation process 1401 as shown in FIG. 14. Then the quantized information D changed, by printing on paper by a printer 1402, to significantly deteriorated on-paper information (print) E. Consequently, the aforementioned extraction of the additional information from the on-paper information for the purpose of preventing forgery corresponds to the extraction of the additional information B from the on-paper information E after the processes shown in FIG. 14.
As the information is significantly changed by the processes 1401, 1402, it is very difficult to embed the additional information in visually easily unrecognizable manner and to correctly extract the embedded information from the information on paper.
Also FIG. 15 shows an example of conventional digital watermarking technology the image information is synthesized for example to a high frequency range after conversion into a frequency range for example by Fourrier transformation instead of the real space area. In the example shown in FIG. 15, the image information is converted into the frequency range by an orthogonal transformation process 1501, and the additional information is added to a visually easily unrecognizable specified frequency by an adder 1502.
After the image information is returned to the real space area by an inverse orthogonal transformation process 1503, there is executed a filtering process involving major changes such as the pseudo gradation process and the printer output as in the case shown in FIG. 14.
FIG. 16 shows a procedure of separation of the additional information from the information on paper. The information of the print is entered by passing the print through an image reading device 1601 such as a scanner. As the printed image has the gradation by the pseudo gradation process, the entered information is subjected to a gradation reconstruction process 1602 which is an inverse pseudo gradation process. The gradation reconstruction process is generally executed with an LPF (low-pass filter). The information after the reconstruction is subjected to an orthogonal transformation process 1603 and the embedded additional information is separated by the power of a specified frequency in a separation process 1604.
As will be apparent from FIGS. 15 and 16, there are involved a number of complex processes from the embedding of the additional information to the extraction thereof. In case of a color image, there is also involved a color conversion process for converting into the colors specific to the printer. In order to achieve satisfactory separation even after such complex processes, there is required to the insertion of a signal of a very high resistance. It is difficult to insert a highly resistant signal while maintaining the satisfactory image quality.
Also since the involved processes are many in number and complex, the process time required for embedding and extraction becomes very long.
Also in the process of the aforementioned Japanese Patent Application Laid-Open No. 7-123244 in which the additional information is embedded in the high frequency range, if the error diffusion method is employed in the subsequent pseudo gradation process, the additional information may be buried in the range of the texture generated by the error diffusion due to the characteristics of the high-pass filter specific to the error diffusion method so that the probability of failure in the extraction becomes very high. Also there is required a highly precise scanner device for the extraction.
Thus, the processes shown in FIGS. 14 and 15 are not adequate in case the pseudo gradation process is to be adopted. Stated differently, there is required an embedding process for the additional information, fully utilizing the characteristics of the pseudo gradation process.
Examples of a process capable of combining the embedding of the additional information and the redundancy of the pseudo gradation process are disclosed in the Japanese Patent Registrations Nos. 2,640,939 and 2,777,800.
The former is to mix data into the image signal, in binarization with a systematic dither process, by selecting one of the dither matrixes representing a same gradation.
However it is difficult to output a photographic high image quality in the systematic dither process, unless employing a printer of a high resolution with a very high mechanical precision, since even a slight aberration in the mechanical precision results in a low frequency noise such as lateral streaks which is easily recognizable on the paper.
Also if the dither matrix is changed periodically, the frequency distribution is distorted by a specified frequency generated by the regularly arranged dither pattern to detrimentally affect the image quality.
Also the gradation representing ability varies significantly according to the kind of the dither matrix. Particularly on paper, the area change rate for example by the overlapping of dots varies depending on the kind of the dither matrix, so that the density may fluctuate by the switching of the dither matrix even in an area where the image signal indicates a uniform density.
Also there may probably result erroneous extraction if the extraction is executed by estimating the dither matrix employed for binarization while the pixel values of the original image information is unclear.
On the other hand, the latter is to employ a color dither pattern process and to embed the additional information according to such color dither pattern. Also in such process, the deterioration of the image quality by switching is inevitable as in the former process.
Also in comparison with the former process, the latter process can embed the additional information of a larger amount, but results in a variation in the color hue resulting from the change in the arrangement of the color components, leading to a deterioration of the image quality particularly in the solid image area.
Also the extraction on paper is anticipated to become more difficult. In any case, these two processes depending on the change of the dither matrix are associated with the drawback that the extraction is difficult while the deterioration of image quality is considerably large.
Therefore, the present applicant already proposed a method capable, utilizing the texture generated by the error diffusion method, of artificially preparing a combination of the quantized values that cannot be generated in the ordinary pseudo gradation process, thereby embedding the additional information.
In this method, the image quality is visually not deteriorated since there is only involved a slight change in the shape of the texture. Also the density in the aerial gradation can be maintained by employing a method of changing the quantization threshold value in the error diffusion method, so that the additional information can be extremely easily embedded.
In the above-described method, however, it is necessary to distinguish whether the texture is artificial or not at the extracting operation. In the print on paper, the texture may not be satisfactorily reproduced by the displacement of image dot from the desired print position for example by an error in the dot placement.
Also in case of a color image, the additional information is embedded principally in a color component of lowest visual sensitivity, but the judgment of texture in the real space area is easily affected by other color components, so that the extraction of the additional information becomes difficult.
There is also known a method, as disclosed in the Japanese Patent Registration No. 2,833,975, of embedding in the image additional information of a very large information amount such as audio information. Such method is to convert the audio information into dot codes known as so-called two-dimensional bar codes and print such codes in a marginal area of the image or in the image itself.
In such method, however, the embedded dot codes deteriorate the aesthetic feature of the image since such dot codes are not embedded in a not easily recognizable manner in the image information.
Though there is described an example of embedding the codes with transparent paint in the image in order to achieve not easily recognizable embedding, such method not only requires transparent ink leading to an increase in the cost but also deteriorates the image quality of the output on paper.